This invention relates generally to a method for producing a hole in a structure and, more particularly, to an improved method of forming a diffuser section of an airfoil cooling hole by way of rapid electro discharge machining (EDM).
Airfoils, such as turbine blades and vanes, are typically exposed to high temperatures ranging from about 800° C. to 1600° C. within a gas turbine engine. One method of protecting airfoils from such extreme temperature conditions includes film cooling. Film cooling comprises the method of passing pressurized air through cooling holes, thereby allowing the air to pass over the exterior of the airfoil as the combustion gases encounter the airfoil. The geometric shape of the cooling holes includes both cylindrical holes and shaped holes. Cylindrical holes comprise holes generally having a circular cross section through the entire exterior wall of the airfoil, thereby allowing the pressurized air to pass through the airfoil. Manufacturing processes used to manufacture cylindrical holes are discussed in U.S. Pat. No. 6,229,113, which is owned by the assignee of the present invention and hereby incorporated herein by reference.
Shaped holes, alternatively, include both a cylindrical meter section and a diffuser section. The cylindrical meter section allows the pressurized air to pass through the airfoil, and the diffuser section assists in directing the pressurized air over the airfoil's exterior surface. In order to direct the pressurized air as close as possible to the exterior surface of the airfoil, the shape of the diffuser section diverges outwardly from the cylindrical meter section to the airfoil's exterior surface. A plurality of shaped holes are simultaneously formed by a “comb type” EDM electrode having a plurality of “teeth” or electrodes that are advanced into the airfoil.
The present method for manufacturing shaped holes is conventional EDM, wherein an appropriately shaped electrode contacts a structure that is typically immersed in a dielectric fluid. Near contact between the electrode and the structure, combined with a pulsed voltage, creates a spark between the electrode and the structure, thereby causing the structure to erode in the shape of the electrode.
A preferred shape for a diffuser section of an airfoil cooling coil is a flared trapezoidal shape that diverges outwardly from the cylindrical meter section. Accordingly, it has been common to use a shaped EDM electrode which is advanced in a single stroke toward the meter section to obtain a shaped diffuser section with smooth, linear walls.
Although effective, the use of a relatively large shaped electrode was a relatively slow process, and production efficiencies and design limitations required that a different method be devised. Accordingly, a rapid EDM process was developed wherein the larger shaped copper electrode was replaced with a relatively tiny solid or hollow copper or brass tube that reciprocates back and forth across the width of the diffuser section as it advances in steps along the length of the diffuser section to form the trapezoidal shaped diffuser section. However, with this process it is impossible to obtain a trapezoidal shaped opening with a smooth linear wall and instead, the smooth wall is replicated by the formation of staggered steps with a gradual off-setting of the steps as the electrode is advanced toward the meter section. Thus, for a diffuser length of 0.100 inches, the electrode is advanced about 100 times so as to form 100 sub-steps along the wall of the diffuser. This number may be adjusted for design purposes.
From a performance standpoint, the diffuser formed by way of the rapid EDM process is substantially equivalent to the diffuser section formed by way of the shaped electrode. In each case, the diversion angle of the diffuser section is limited to about 10°. If a diffuser is formed having angles greater then 10°, which is desirable for purposes of effective cooling, then the air flowing through the diffuser section will separate from the boundary walls thereof, resulting in inefficient and inadequate cooling of the airfoil.